1. Field of the Invention
The present invention relates in general to a method of forming a photoresist pattern free from side-lobe phenomenon, and more particularly to a method of forming a photoresist pattern free from side-lobe phenomenon by improving the design of the mask.
2. Description of the Related Art
Optical lithography has advanced in recent years in its ability to produce very small features. The most important recent development in lithography was the introduction of a photomask structure called a phase shift mask (PSM). Phase shift masks enable compensation for the diffraction effects which otherwise limit the size of the smallest features imaged by optical lithography method.
Optical lithography refers generally to the technology which enables etching patterns on a substrate through use images being developed onto the substrate by a mask. Generally, the process involves directing light (such as ultraviolet light) through a photomask to expose a light-sensitive film previously deposited on the substrate. If the lightsensitive film is a so-called onto the substrate by a mask. Generally, the process involves directing light (such as ultraviolet light) through a photomask to expose a light-sensitive film previously deposited on the substrate. If the lightsensitive film is a so-called positive resist and the resist is located beneath a clear area in the photomask, the resist undergoes a physical and chemical change that renders it soluble in a development solution. This process results in the transfer of an image from the photomask to the resist film. Finally, the application of an acid to the surface of the resist film transfers the image on the resist film to the surface of the substrate.
FIG. 1(a) to FIG. 1(d) illustrate how the mask pattern, the light transmittance, the light intensity, and the developed photoresist related to each other according to a conventional method. A mask 10, as shown in FIG. 1(a), is used to selectively expose portions of a wafer 20 covered with a photoresist layer 26 shown in FIG. 1(d). The mask 10 includes a dark region 12, a partial-clear region 14, and a clear region 16. Light 30 transmits through the mask 10 and exposes the photoresist layer 26 on the wafer 20.
The transmittance of the mask 10 is plotted in FIG. 1(b), wherein the transmittances of the dark region 12, the partial-clear region 14, and the clear region 16 are 0%, 6%, and 100%, respectively.
FIG. 1(c) illustrates the intensity of the light exposed onto the photoresist layer 26. The light exposure profile has a main peak 42 corresponding to the center of the clear region 16. Away from the main which is indicated by the horizontal dashed line.
FIG. 1(d) illustrates the developed photoresist layer 26 after exposure by using the mask 10 of FIG. 1(a). The undesired side-lobe hole 28 is formed in the photoresist layer 26, along with the predetermined hole 24. The predetermined hole 24, which corresponds to the clear region 16 on the mask 10, is used to create a contact hole in the layer under the photoresist layer 26. The undesired side-lobe hole 28 causes a side-lobe phenomenon in the layer under the photoresist layer 26, which usually impedes the process window.
It is necessary to propose a method or design on the mask to suppress the side-lobe phenomenon.